1. Field of the Invention
The present invention relates to an improvement of a tuning circuit using a negative resistance circuit in order to improve Q of the tuning circuit.
2. Description of the Related Art
In a radio receiver, in order to improve frequency selectivity of a high frequency section thereof, there are methods to add a tuning amplifying section thereto or to use a negative resistance circuit. Further, the latter method contains a so-called reproduce receiving method wherein positive feedback is applied from an output of an amplifier to a tuning circuit section and a method to improve effective Q of a tuning circuit by adding a negative resistance circuit including a negative impedance converter, etc. to a tuning circuit. However, in the positive feedback method, since it is necessary to keep a circuit always in oscillatable condition even if a tuning frequency is changed, this is not easy. Also, in the method using the negative resistance circuit, circuit construction for it is simple and operates stably in a high frequency band and setting of negative resistance value is simple but a negative resistance circuit having such circuit construction is unknown.
An object of the present invention is to provide a tuning circuit using a negative resistance circuit wherein path band width and circuit gain are constant and it is easy to set a negative resistance value even if a tuning frequency is changed, and circuit construction of the negative resistance circuit is simple.
In order to attain the above object the present invention provides a tuning circuit comprising a series resonance circuit, a driving circuit connected to one side of the series resonance circuit and a negative resonance circuit connected to other side of the series resonance circuit, said negative resistance circuit consisting of a differential amplifying circuit connected to the series resonance circuit and a low output impedance circuit connected to the differential amplifying circuit, an output of the low output impedance circuit being fed back directly to a same phase input side of the differential amplifying circuit and to an inverse phase side thereof through a negative resistance value setting resistor, and a tuned output signal being taken out from a same phase output side of the differential amplifying circuit.
In the present invention, said differential amplifying circuit may include two transistors emitters of which are connected with each other in common and the low output impedance circuit may be an emitter follower circuit.